The use of hematopoietic stem cells for bone marrow transplantation has revolutionized the approaches used to treat a large number of hematological malignancies (e.g., leukemias), as well as several widespread autoimmune diseases, and is a critical treatment for immunodeficiencies (Buckley, R H, Annu Rev Immunol 22, 625-655, 2004). The use of hematopoietic stem cell transplantations has also been successful in mitigating the effects of exposure to high levels of radiation in several instances (Bishop, M R, Stem Cells 15 Suppl 2, 305-310, 1997). In addition, hematopoietic stem cell transplantations have been used to enable the administration of high doses of cytotoxic chemotherapeutic agents to patients who suffer from a number of solid organ tumors, thus enabling the repopulation of the bone marrow following drug-induced toxicity (Crivellari, G et al., Oncologist 12, 79-89, 2007). The use of hematopoietic stem cell transplantation to improve the rate of engraftment of solid organ transplantations is another recent application of this medical procedure (Delis, S et al., Pancreas 32, 1-8, 2006). Recent studies also indicate that bone marrow transplantation may have value in the treatment of heart disease (Engelmann, M G et al., Curr Opin Mol Ther 8, 396-414, 2006). Although the basis of this effect is unknown, these findings raise the possibility that hematopoietic stem cells may be reprogrammed to give rise to other tissues (Kiel, M J et al., Dev Biol 283, 29-39, 2005). Accordingly, adult hematopoietic stem cells may have a much broader utility than, and may provide an alternative to, controversial embryonic stem cell therapy. These therapeutic applications of hematopoietic stem cell transplantation demonstrate the medical and economic impact of improving hematopoietic stem cell transplantation.
Several problems have limited the therapeutic application of hematopoietic stem cell (HSC) transplantation. For example, one major problem is the low number of HSCs available for transplants. Patients who suffer from bone marrow failure, autoimmune diseases, congenital immunodeficiencies, or hematological malignancies do not provide a good source of HSCs for autologous transplantation (Linker, C, Best Pract Res Clin Haematol 20, 77-84, 2007). In addition, some bone marrow failure patients and cancer patients require multiple rounds of HSC transplantation in order to achieve full HSC engraftment following radiation or chemotherapy treatment (Oliansky, D M et al., Biol Blood Marrow Transplant 13, 1-25, 2007). In cases where autologous HSC transplantation is not possible, there is the additional problem of identifying an appropriately histocompatible bone marrow donor. This is generally accomplished using registries that have enrolled more than 6 million potential donors (de Mello, A N et al., Telemed. Telecare 12 Supp13, 64-6, 2006). Additionally, once selected, the donor must undergo a grueling and painful process to mobilize HSCs into the blood followed by 4-5 days of leukapheresis to isolate rare long-term HSCs (Nervi, B et al., J Cell Biochem 99, 690-705, 2006). There have been a number of novel approaches aimed at solving the problem of low numbers of HSCs available for transplants, by expanding HSCs ex vivo after isolation, but the ability to generate large numbers of long-term repopulating HSCs that remain available and bioactive over a period of years has remained elusive (Hoffman, R, Curr Opin Hematol 6, 184-91, 1999).
Another problem that limits the therapeutic application of HSC transplantation is the time required for the transplanted HSCs to reconstitute the functional and mature hematopoietic lineages (i.e., the hematopoietic compartment) after the ablation of the transplant recipient's resident immune system. One of the elements required to promote the successful engraftment of transplanted HSCs is the removal of the resident immune system. This is routinely done by total body irradiation or chemical ablation. The end result of this process is an almost completely immunocompromised patient that is highly susceptible to opportunistic infection by environmental microorganisms that humans routinely interact with during normal activities, such as breathing and eating. This problem is of greater concern with the significant increase in the variety and heterogeneity of iatrogenic infectious agents, many of which are highly resistant to existing antibiotics. The time required for recovery of mature hematopoietic lineages after an HSC transplant significantly affects the risk of the transplant recipient developing and ultimately succumbing to opportunistic infections.
The time required to repopulate mature hematopoietic lineages in an HSC transplant recipient is affected by several variables. One such variable is the differing recovery times required to repopulate the different hematopoietic lineages following an HSC transplant. For example, the myeloid compartment, which is composed of monocytes, neutrophils, and basophils, usually requires 4-8 weeks to recover following HSC transplantation. The lymphoid compartment requires a significantly longer recovery time in humans. For example, T-cells, NK cells, and NKT-cells require between 4-8 months to recover, while B-cells require over 12 months to recover in most individuals. The recovery time of the myeloid lineage cells is critical for the minimal required defenses against food-borne and environmental microorganisms. Another variable affecting the time required to repopulate mature hematopoietic lineages in an HSC transplant recipient is the number of HSCs available for transplantation and the size/weight of the recipient. A further variable is the nature of other treatments that an HSC transplant patient may have been subjected to prior to the transplantation. In most patients with some form of cancer, the patient will have been given several rounds of chemotherapy prior to receiving an HSC transplant. The use of such cytotoxic drugs can impact the bone marrow niches to which the transplanted HSCs home to and begin the differentiation process. In some instances where the niches have been destroyed by cytotoxic drugs, several HSC transplants may be required to initially reconstitute the niches and subsequently seed the niches with pluripotent HSCs.
Moreover, the recovery time of mature hematopoietic lineages following HSC transplantation is largely dependent of the ability of the donor HSCs to find their way to bone marrow niches. Once the HSCs arrive at the bone marrow niches, they need to establish a molecular crosstalk with the niche-resident cells. This cross talk is thought to regulate the nature and levels of cell-intrinsic signals within the HSCs that regulate their survival, proliferation, self-renewal, and differentiation. Thus, failure of HSCs to find their way to bone marrow niches, home properly, or correctly establish such molecular crosstalk with the niches can result in HSC engraftment failure. Additionally, failure of HSCs to find their way to bone marrow niches can also result in only a short term recovery of mature hematopoietic lineages or only a partial reconstitution of mature hematopoietic lineages that will slowly subside as a result of long-term bone marrow failure.
The lag time between ablation of a patient's resident immune system and hematopoietic lineage reconstitution by HSC transplantation is thus one of the major risk factors for the development of potentially fatal complications, such as opportunistic infections or HSC engraftment failure. Several approaches have been attempted to decrease the time required to reconstitute mature hematopoietic lineages following HSC transplantation. Examples of such approaches include the use of higher numbers of HSCs for transplantation, partial resident immune system ablation and multiple transplants of smaller numbers of HSCs, pre-conditioning of donor bone marrow to retain its resident T-cells, growth factor treatment of the patient following HSC transplantation, and the use of monoclonal antibodies and/or small molecule modifiers that target enzymes affecting E-selectin expression in order to improve HSC homing to bone marrow niches after HSC transplantation (Adams G B and Scadden D T, Gene Ther 15, 96-99, 2008; Campbell T B and Broxmeyer H E, Front. Biosc. 13, 1795-1805, 2008; Rocha V and Boxmeyer H, Biology of Bone marrow transplantation 16, S126-S132, 2009; and Hogatt J. et al., Blood 113, 5444-55, 2009). However, these solutions only provide a modest improvement over current approaches without significantly affecting the frequency of fatal opportunistic infections in the patient population.